The production of mixed fertilizer products has been performed by mixing and/or reacting of the individual fertilizer components and forming the components/products into particles of the desired size range. The particles can comprise a range of sizes, representing a less-preferred product, and are screened, with off-size material separated and collected for reprocessing or to be sold as a less valuable product. Blended fertilizers are also common where the fertilizer components are mixed but not formed into particles containing all of the components, or nutrients. This leads to non-homogeneous particles with differing component content, in particular the amount of nitrogen can vary throughout the mixture. The nonuniformity leads to a nonuniformity in the spreading, time-release rates and effectiveness of the nutrients.
Fertilizers that have been formed into pellets or granules containing mixtures of the nutrients can also include trace compounds that are added for the enhancement of the fertilizer for a particular use. In addition, fertilizers that are formed into uniform particles can have more control over the release of nutrients into the soil over which the fertilizers are spread. The rates of release of the nutrients and the control of the composition can reduce the loss of the nutrients before plants have the ability to absorb them.
Prilling is one method of the production of small spherical pellets, wherein the melted material is passed through an apparatus called a prill head which generates streams of liquid droplets. The droplets have little or no volatile matter, and contain no solvents to be removed. The droplets are cooled with a continuous stream of cooling gas, typically air to solidify the droplets. Granulation is another method, whereby melted material is discharged through spray nozzles to create particles of the fertilizer that are mixed and agglomerated in a rotating drum.
However, both prilling and granulation methods have drawbacks. In granulation the particles produced lack uniformity (size and shape), and generally require reprocessing of a significant fraction of off-spec material, adding to expense through need for reprocessing equipment (screen, grinders, conveyors) and dust and sub-micrometer particulate capture equipment. In prilling, particles produced have better size and shape uniformity than with granulation but large quantities of air must be treated to capture sub-micrometer particulates before venting to the atmosphere.